LM3402 datasheet(21/38 Pages) TI

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VIN

LM3402

GND

80V

VIN

Module

Connector

Module

Connector

TVS

CIN

Board Trace

Inductance

LM3402, LM3402HV

www.ti.com

SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

VIN PIN PROTECTION

The VIN pin also has an ESD structure from the pin to GND with a breakdown voltage of approximately 80V. Any

transient that exceeds this voltage may damage the device. Although transient absorption is usually present at

the front end of a switching converter circuit, damage to the VIN pin can still occur.

When VIN is hot swapped in, the current that rushes in to charge CIN up to the VIN value also charges (energizes)

the circuit board trace inductance as shown in Figure 28. The excited trace inductance then resonates with the

input capacitance (similar to an under-damped LC tank circuit) and causes voltages at the VIN pin to rise well in

excess of both VIN and the voltage at the module input connector as clamped by the input TVS. If the resonating

voltage at the VIN pin exceeds the 80V breakdown voltage of the ESD structure, the ESD structure will activate

and then “snap-back” to a lower voltage due to its inherent design. If this lower snap-back voltage is less than

the applied nominal VIN voltage, then significant current will flow through the ESD structure resulting in the IC

being damaged.

An additional TVS or small zener diode should be placed as close as possible to the VIN pins of each IC on the

board, in parallel with the input capacitor as shown in Figure 29. A minor amount of series resistance in the input

line would also help, but would lower overall conversion efficiency. For this reason, NTC resistors are often used

as inrush limiters instead.

Figure 28. VIN Pin with Typical Input Protection

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LM3402 Datasheet(PDF) 21 Page - Texas Instruments